A spike protein is a structure that protrudes from the surface of certain viruses and is a defining feature of the viral envelope, the outer layer enclosing the virus’s genetic material. Spike proteins act as a tool, allowing the virus to attach to a host cell and initiate an infection.
Function in Viral Entry
The primary role of the spike protein is to facilitate a virus’s entry into a host cell, using a “lock-and-key” model. In the case of SARS-CoV-2, the virus that causes COVID-19, the spike protein acts as the key. It is specifically shaped to fit a particular lock on human cells, a receptor protein called angiotensin-converting enzyme 2 (ACE2). The ACE2 receptor is found on various cells, including those in the lungs.
When the virus encounters a human cell, the receptor-binding domain (RBD) of its spike protein connects with an ACE2 receptor, which triggers structural changes in the protein. For the virus to enter the cell, the spike protein must be cleaved, or cut, at two specific sites. This cutting process, often carried out by host cell enzymes like TMPRSS2, allows the viral membrane to fuse with the host cell’s membrane.
Once fusion occurs, a pathway is created for the virus to inject its genetic material into the cell. This genetic material then takes over the cell’s machinery, forcing it to produce new copies of the virus. The process is highly specific, as the precise fit between the spike protein and the ACE2 receptor determines which species and cell types the virus can infect.
Application in Vaccine Technology
Scientists have utilized the spike protein to develop vaccines, particularly mRNA vaccines. These vaccines work by providing the body with genetic instructions (mRNA) that direct our own cells to manufacture a harmless version of the spike protein. Only the instructions for the spike protein are provided, not the entire virus, so a viral infection cannot occur from the vaccine.
Once the mRNA from the vaccine enters muscle cells near the injection site, the cell’s machinery reads the instructions and produces spike proteins. These proteins are then presented on the surface of the cells. The immune system recognizes these spike proteins as foreign and initiates a protective immune response without an actual infection.
The presence of these vaccine-induced spike proteins stimulates the immune system to create antibodies specifically designed to target them. In addition to antibodies, the body also generates memory cells. These specialized immune cells “remember” the shape of the spike protein, enabling a faster and more effective response if the body ever encounters the actual virus in the future. This process prepares the immune system to neutralize the virus before it can cause widespread infection.
Spike Protein Safety and Persistence
Concerns have been raised about the safety of the spike protein from vaccines. There are distinct differences between the spike protein from vaccination and that from a natural infection. A primary distinction is location; vaccine-induced spike proteins are mainly produced at the injection site and in nearby lymph nodes. In contrast, a viral infection produces spike proteins throughout the body as the virus replicates.
The quantity and duration of the protein’s presence also differ. After vaccination, the body produces a limited, temporary amount of the protein. The mRNA instructions break down within days, and the resulting spike proteins are cleared by the immune system within a few weeks. An active infection continuously generates spike proteins, leading to higher and more prolonged exposure.
The spike protein from vaccines is also structurally different from its viral counterpart. Scientists engineered the vaccine’s spike protein to remain in a stable, pre-fusion conformation. This modification creates a more consistent target for the immune system. This stabilized form is less capable of fusing with cell membranes compared to the version on the live virus.